Through air drying has become the technology of preference for making tissue for many manufacturers who build new tissue machines as, on balance, through air drying ("TAD") offers many economic benefits as compared to the older technique of conventional wet-pressing ("CWP"). With through air drying, it is possible to produce a single ply tissue with good initial softness and bulk as it leaves the tissue machine.
In the older wet pressing method, to produce a premium quality tissue, it has normally been preferred to combine two thin plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, embossing two plies together imposes marked economic penalties which can be avoided in production of a one-ply product using through air drying. But even though through air drying has been preferred for new machines, conventional wet pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air. It is not normally economic to convert older CWP tissue machines to TAD. Further, single ply machines can normally run at high speeds.
What has been needed in the art is a method of making a premium quality or near premium quality single ply tissue using conventional wet pressing. In this way, advantages of each technology could be combined so older CWP machines can be used to produce high quality single ply tissue at costs which are far lower than those associated with embossing two plies together.
One of the more significant barriers to production of a single ply CWP tissue has been the extreme sidedness of single ply webs using technology known prior to this invention. TAD processes can produce a nice soft bulky sheet having fairly low strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable.
We have found that we can produce a soft high strength CWP tissue with low sidedness by judicious combination of several techniques as described herein. Basically, these techniques fall into four categories: (i) fiber stratification; (ii) chemical stratification; (iii) low angle, high adhesion creping; and (iv) reverse embossing. Of these four techniques, the first two seem to be more flexible and exhibit more pronounced benefits than the latter two, but by various combinations of these techniques as described, taught and exemplified herein, it is possible to almost "dial in" the required degree of sidedness depending upon the desired goals.
CWP processes can be carried out on fourdrinier, twin wire, suction breast roll, and crescent forming machines. Energy consumption is lower and the production speeds can be considerably higher than those used on TAD machines. The plies previously produced on CWP machines are usually fairly strong but, as mentioned, they have a distinctly two-sided character; consequently, CWP is most commonly used for two-ply products so that the softer sides of each ply can be positioned on the exterior of each sheet and the harsher surfaces buried in the interior, each facing the other. However, there is a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Further, CWP plies in a multi-ply structure need to be embossed to bond the plies together and help restore some of the bulk squeezed out in the pressing operation used to dewater each ply. For these reasons, many single-ply CWP products currently found in the marketplace are typically low end products. These products often are considered deficient in thickness, softness, and exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in "away from home" applications in which the person buying the tissue is not the user.
1. Field of The Invention
The present invention is directed to a soft, single-ply bulky tissue paper having low sidedness and processes for the manufacture of such tissue.
2. Description of Background Art
Paper is generally manufactured by suspending cellulosic fiber of appropriate geometric dimensions in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds adds to and increases the perceived softness of resulting bathroom tissue product.
However, creping alone may not be sufficient to impart the optimum degree of softness to the bathroom tissue. Therefore, as related by Soerens et al. in U.S. Pat. No. 4,795,530, compounds such as quaternary amines that function as debonding agents are often incorporated into the paper web. As Soerens points out, cationic quaternary amines can be added to the initial fibrous slurry from which the paper web is subsequently made. Soerens teaches that it is preferable, however, to spray the chemical debonding agent onto the cellulosic web, after it is formed but before it is dried, and describes a method for spraying the amines onto the partially dewatered web in such a way that it is alleged the amines penetrate no more than 40% of the way through the thickness of the web leaving the remainder of the thickness "effectively untreated".
One-ply bathroom tissue generally suffers from the problem of "sidedness"--that is, one side of the sheet is generally perceived as being appreciably less soft than the other side. Sidedness is introduced into the sheet during the manufacturing process. The side of the sheet that was adhered to the Yankee and creped off i.e., the Yankee side is generally softer than the "air" side of the sheet. This two-sidedness is seen both in sheets that have been pressed to remove water and in unpressed sheets that have been subjected to vacuum and hot air (through-drying) prior to being adhered to the crepe dryer. The sidedness is present even after treatment with a softener. An acceptable one-ply tissue should not only be soft and strong but should also exhibit softness of each side approaching the softness of the other. The prior CWP art has been unable to solve this problem.
The most pertinent prior art patents will be discussed but, in our view, none of them can be fairly said to apply to reduction of sidedness in one-ply tissue nor to teach or make obvious use of combinations of the four basic techniques described above for reduction of sidedness.
The Furman et al. U.S. Pat. No. 5,187,219 discloses a polyacrylamide creping adhesive. The Grossman U.S. Pat. No. 4,063,995 discloses a four-component creping adhesive. The Knight et al. U.S. Pat. No. 5,234,547 discloses polyacrylamide as a creping aid.
The Ampulski et al. U.S. Pat. No. 5,164,046 and Publication WO 09302252 disclose a creping angle of 83.degree.. Polyvinyl alcohol is the creping adhesive. The Edwards et al. U.S. Pat. No. 4,894,118 discloses use of a creping angle between 60-100 degrees and 70-80 degrees but for recreped absorbent products. The Klowak U.S. Pat. Nos. 4,448,638 and 4,482,429 assigned to. the Assignee herein disclose creping angles between 52-72.degree. using a reverse creping blade.
The Awofeso et al. U.S. Pat. Nos. 5,087,324 and 5,164,045 assigned to the Assignee herein disclose stratified paper webs having a first layer of anfractuous fiber, chemithermomechanical pulp and softwood kraft and a second layer of eucalyptus. The Spendel U.S. Pat. Nos. 4,959,125 and 4,940,513 and the Ampulski et al. U.S. Pat. No. 5,164,046 disclose methods of producing one-ply tissue paper consisting of spraying starch and surfactant on the tissue. No distinction is shown on which side the starch and surfactant are sprayed. The Ampulski patent indicates that these components are sprayed on both sides. The WO 09302252 publication discloses a method of making single-ply or double-ply tissue by spraying starch and surfactant on both sides of the web. Lim WO 82/00485 publication discloses a process for spraying an acidified debonder on the sheet while on the forming fabric before vacuum dewatering. Many studies disclose the use of debonders and softeners to improve softness. The following are representative prior art references: Freimark et al. U.S. Pat. No. 3,755,220, Aug. 28, 1973; Shaw et al. U.S. Pat. No. 3,821,068, Jun. 28, 1974; Harvey et al. U.S. Pat. No. 3,554,802, Jan. 12, 1991; Emanuelsson et al. U.S. Pat. No. 4,144,122, Mar. 13, 1979; and Becker et al. U.S. Pat. No. 4,158,594, Jan. 19, 1979. None of the foregoing prior art references relate to one-ply tissue having a low sidedness and exhibiting a sidedness parameter of less than 0.3 along with a tensile modulus of no more than 32 grams/percent strain; a GM MMD friction of no more than about 0.23; and a cross directional dry tensile strength of at least 200 grams per 3 inches.